Plasmon resonance sensor

The invention relates to a plasmon resonance sensor, in particular for biosensor technology, comprising a light-transmissive body, a—applied to an area of the body—reflective metal layer or semiconductor layer having a surface which can be sensitized to molecules to be detected, which comprises partial areas arranged alongside one another in a series and serving for forming a respective one of a plurality of measurement cells, comprising an areal light source for producing a beam path incident on the inner side of the layer through the body, and comprising a detector, which captures the reflected emergent beam path and ascertains in a time-dependent manner the angle of incidence of the light which changes as a result of molecular attachments to the sensitive surface and in the case of which an intensity minimum of emergent light occurs owing to resonance, a Fourier lens being arranged in the emergent beam path and the detector being arranged at the focal point distance with respect to the Fourier lens.

The phenomenon of surface plasmon resonance (SPR) involves a collective excitation of the electrons at the surface of a layer having free electrons. The resonant frequency of the surface plasmons is highly sensitive to the refractive index of the medium adjacent to the sensitive surface. This can be used to measure thin layers with regard to the refractive index or the layer thickness (up to approximately one light wavelength). In biosensor technology, in particular, this effect is used to examine the kinetics of attachment of biomolecules from a sample liquid to a functionalized metal surface. For this purpose, the resonance condition of the surface plasmons is detected in time-resolved fashion. The surface plasmons of the thin metal layer are excited by light that falls onto the metal layer at a specific angle or angular range. The resonance condition is then met for a specific combination of wavelength and angle of incidence. Under this resonance condition, the intensity of the light reflected at the metal layer is significantly reduced on account of the generation of surface plasmons. In order to find the resonance condition, either the angle of incidence (given a constant wavelength) or the wavelength (given a constant angle of incidence) can be tuned and the intensity of the reflected light can be detected. A gold-coated glass body (prism) and light having a constant wavelength, which impinges on the gold layer at different angles of incidence, are generally employed.

It is known from EP 305 109 B1 in the case of angle-resolved measurement, to produce the corresponding angular range optically by means of a beam fan which is focused onto the metal layer by means of a hemispherical glass lens. The focusing enables only a single measurement, after which the layer which is sensitive to molecules has to be regenerated again before a further measurement becomes possible. Moreover, owing to the focusing, there is the risk of local heating of the metal layer and, as a result of this, the possibility of corruption of the measurement values.

DE 100 23 363 C1 has already disclosed realizing different angles of incidence of the light by irradiating the metal layer with a divergent light beam which issues from a point light source in the form of a laser diode. In this case, simultaneous multiple measurements of different samples are made possible by virtue of the fact that the incident light is fed divergently only in one direction (in the incidence plane), while it is collimated by means of a cylindrical lens in the direction perpendicular thereto, a plurality of measurement cells being arranged in a series transversely with respect to the incidence plane on the metal layer. Harmful heating of the metal layer is prevented owing to the divergent irradiation. Within the incidence plane or divergence plane, different angles of incidence are present in each case at a different location of the metal layer. This leads to an influencing of the measurement values owing to inhomogeneities of the metal layer. Furthermore, the still divergently emergent beam path is detected directly, which necessitates large detectors that are each assigned to a single measurement cell. This leads to a complicated and spatially extended device which enables only a small number of measurement cells and thereby simultaneous measurements.

DE 100 55 655 C2 discloses the plasmon resonance sensor comprising a plurality of measurement cells as described in the introduction. In this case, each measurement cell or partial area of the metal layer is assigned a dedicated light source formed by an optical waveguide having an extended and not point-type emission area, the optical waveguides being directly connected to the light-transmissive body and a Fourier lens and a cylindrical lens being arranged in the emergent beam path, which permits a common detector. The areal light sources, which are in each case comparable to a multiplicity of adjacent point light sources, have the consequence that the range of different angles of incidence is present at all locations of the metal layer. The Fourier lens is arranged for the purposes of a Fourier imaging of a 2 f arrangement between the emission areas of the light sources and the detector, the emergence optical unit ensuring that identical angles of incidence are combined on the detector and are separated there from light based on other angles of incidence. It is thus possible to obtain exact averaged measurement results which are free from unfavorable influences due to heating or inhomogeneities of the metal layer.

What is disadvantageous about this known plasmon resonance sensor, however, is that each measurement cell or partial area of the metal layer is assigned a dedicated areal light source, which greatly limits the possible number of measurement cells. Moreover it is not easy to obtain an angular spectrum which encompasses the entire angular range to be measured and which is dependent on the distance between the optical fiber and the metal surface, the numerical aperture of the fiber and the fiber diameter. In addition, the divergent emission of the metal layer requires a comparatively extended optical unit with a large Fourier lens which captures the emergent beam path. That, too, is detrimental to a compact and uncomplicated design.

Accordingly, the invention is based on the object of improving this plasmon resonance sensor such that, whilst maintaining the advantages outlined, it is simpler and more compact and has greater performance.

Proceeding from the sensor described in the introduction, this object is achieved according to the invention by virtue of the fact that an incidence optical unit is provided between the light source and the layer, and it collimates the incident beam path in the series direction of the partial areas and directs it onto the layer convergently in the incidence plane running perpendicular to the series direction of the partial areas, such that the single areal light source is imaged on each partial area of the layer in the form of a luminous line.

The convergent irradiation of the metal layer enables a compact design with small components or optical elements both on the light incidence side and on the light emergence side, in which case very narrow partial areas which are closely adjacent to one another can also be employed in the interests of a large number of measurement cells.

Expedient configurations and developments of the invention are apparent from the dependent claims. In this case, in particular the use of a chip composed of a light-transmissive polymeric plastic as light-transmissive body contributes to simplification and increase in performance since it can be produced inexpensively and simultaneously by corresponding shaping also with optical functions for the light guiding on the emergence side, and if appropriate also on the incidence side, by way of injection molding, as is known in principle from DE 103 24 973 B4. This affords the possibility of designing the chip as a replaceable disposable article, such that regeneration measures become superfluous and not only can a plurality of measurements be carried out simultaneously but the multiple measurements can also be performed in rapid succession. A comparatively expensive optical component is obviated as a result of the integration of the collimation lens into the chip. Furthermore, the insertion of the chip into the plasmon resonance sensor results in an optical coupling between the illumination optics and the detection optics without a disturbance-prone coupling by means of immersion media.